I ride in southern Wisconsin and like to ride down to at least 20 F.
Last winter I bought a Giro skiing/snowboarding helmet with a pair of goggles. It is much, much more comfortable and warmer than a balaclava under a regular helmet. With goggles, I need to only protect the lower part of my face. Since I let my beard grow longer in the winter, I need to add that only below 25 F.
At warmer temperatures, 30-40 F, my regular helmet with a rain cap and skull cap is often enough.